Low-pressure mercury vapor arc discharge lamps are radiation sources which are used on a very large scale both for general illumination and for special purposes (e.g., photochemistry), because they convert the applied electric power very efficiently into radiation. In general these lamps consist of a sealed tubular envelope which may be straight or curved, for example, bent to form a circle or U-shaped. The envelope contains an ionizable medium which includes a gas mixture of mercury and one or more rare gasses in which a positive discharge column is produced. Means are present for maintaining this positive discharge column by supplying electric energy to the gas mixture. The means usually comprise two electrodes. Mainly ultraviolet radiation is produced in the discharge, the greatest part having wavelengths of approximately 2537 angstrom. The ultraviolet radiation is converted by means of a phosphor layer disposed on the internal surface of the lamp envelope, into radiation having waves of a longer length and a spectral distribution, depending on the phosphor material used, in the near ultraviolet or in the visible part of the spectrum.
Magnetic fields have been used with compact fluorescent lamps for use in incandescent fixtures as well as conventional and non-conventional elongated, tubular-shaped fluorescent lamps for various reasons. For example, U.S. Pat. No. 4,187,446, which issued to Gross et al on Feb. 5, 1980 and U.S. Pat. No. 4,311,942, which issued to Skeist et al on Jan. 19, 1982 disclose the use of an alternating, non-constant, electromagnetic field generated by a specially designed ballast to spread the arc periodically throughout the volume of a compact fluorescent lamp. U.S. Pat. No. 4,311,943, which issued to Gross et al on Jan. 19, 1982, combines the use of a recombination structure of fine fibers interposed in the arc path with an arc spreading coil which serves as all or part of the ballast of the fluorescent lamp. Since the ballast field is approximately 90 degrees out of phase with the current and light output, B is proportional to di/dt, thus the maximum ballast magnetic field occurs near zero light output which may not be optimum. Furthermore, practical ballast fields generated are relatively low and generally range in the order of 20 to 40 gauss. Additionally, generation of many ballast fields via coil windings require substantial changes in ballast design and may not be compatible with certain advanced high-frequency ballast designs.
U.S. Pat. No. 4,434,385, which issued to Touhou et al on Feb. 28, 1984 is still another patent using magnetic fields with fluorescent lamp. More specifically, this patent suggests the use of a magnetic field locally disposed around a non-conventional lamp for varying the light distribution direction and/or color of the lamp.
U.S. Pat. No. 4,417,172, which issued to Touhou et al on Nov. 22, 1983, relates primarily to suppressing low temperature flickering phenomena caused by moving striation in conventional fluorescent lamps by means of electromagnets or permanent magnets. The teachings of this patent are incorporated herein by reference. The field strengths suggested are chosen in a particular limit to stop the flickering within a desired time and to ensure the easiness of the starting under the relatively severe conditions of an ambient temperature of 0.degree. C. and the power source voltage anticipated by the apparatus. More specifically, this reference teaches limiting the magnetic flux density Y at the center of a transverse section of the discharge tube operating at 0.degree. C. to a value such that Y is less than 600.times.+70. The value X is equal to the quotient obtained by dividing the weighted mean value of the atomic weight of rare gas atoms in the discharge lamp envelope by a product of the pressure value in the lamp, the square of the value of the inner radius of the envelope, and the length of the envelope.
It has been discovered that an increase of approximately 30 percent in the ultraviolet output from a low-pressure arc discharge lamp can be achieved when magnetic fields of higher field strengths are employed at ambient temperatures greater than 25.degree. C. (77.degree. F.). It is known that at ambient temperature above about 25.degree. C. (77.degree. F.), the flickering phenomena is much less a problem than at 0.degree. C. (32.degree. F.). At ambient temperatures of 40.degree. C. (104.degree. F.) and higher normally encountered when a fluorescent lamp is installed in, for example, an enclosed wrap-around fixture, flickering is essentially non-existent.